Heartworm in dogs is caused by the filarial nematode parasite Dirofilaria immitis (DI). Infections by this parasite have reached epidemic proportions in dogs and pose a health threat to man. One of the major problems in treatment is the identification of infections where microfilaria cannot be detected by parasitological examination of blood. The goal of the proposed research is to develop a sensitive and specific monoclonal antibody diagnostic assay to quantitate DI infections. We have identified a major surface polypeptide which appears to be sloughed by living DI parasites into the circulation of all animals tested. The quantity of this peptide in circulation should be an indicator of DI in occult or non-microfilarimic infections. An enzyme linked immunosorbent assay (ELISA) using monoclonal antibodies against this DI antigen would provide a simple clinical assay. In Phase I, we will develop hybridoma cell lines that produce monoclonal antibodies against the purified DI surface polypeptide. These monoclonal antibodies will be incorporated into an immunoassay to detect circulating DI-antigen. In Phase II, we will assess the clinical applicability of this molecular diagnostic system.